Doxorubicin (DOX) is an anthracycline antibiotic used in the treatment of a broad spectrum of human cancers, including acute leukemia, lymphomas, stomach, breast and ovarian cancers. Unfortunately, the clinical use of this highly efficacious anticancer drug is limited due to the development of cardiotoxicity in patients. Doxorubicin- induced cardiotoxicity is a debilitating condition that promotes the onset of congestive heart failure, resulting in reduced quality of life and increased morbidity. While the mechanisms responsible for DOX-induced cardiac dysfunction are unclear, it is well known that the incidence of cardiac dysfunction greatly correlates to the concentration of DOX taken up by the heart. DOX accumulates rapidly within cardiac tissue following exposure, where it preferentially localizes to the mitochondria and promotes free radical production. Elevated free radical production in the mitochondria can lead to severe damaging events resulting in cell death, and evidence suggests that prevention of mitochondrial dysfunction is sufficient to attenuate the cardiotoxic effects of DOX. Therefore, elucidating ways in which the mitochondrial accumulation of DOX can be reduced could result in the development of a therapeutic approach to mitigate the cardiotoxic effects of DOX. In this regard, we recently discovered that endurance exercise training prior to DOX treatment is sufficient to reduce the mitochondrial accumulation of DOX and preserve cardiac function. While the mechanisms responsible for the exercise-induced reduction in the levels of cardiac mitochondrial DOX are unknown, we hypothesize that activity-induced increases in the expression of xenobiotic transport proteins are required. Specifically, the ATP-binding cassette (ABC) transporters are a class of proteins with the capability of facilitating the efflux of chemotherapeutics from the heart. Moreover, four mitochondria-localized ABC transporters are expressed in the heart (i.e. ABCB6, ABCB7, ABCB8 and ABCB10), all of which are upregulated with exercise. Therefore, the goal of this proposal is to establish the effects of these transport proteins in mediating the exercise-induced extrusion of DOX from the heart, and to determine their therapeutic potential to prevent DOX-induced cardiac dysfunction. We will accomplish this by testing the following specific aims: Specific Aim 1) will determine if exercise-induced protection against DOX toxicity is dependent on increased levels of mitochondria-localized ABC transporters; and Specific Aim 2) will determine if overexpression of mitochondrial ABC transport proteins in the heart is sufficient to reduce cardiac DOX accumulation and prevent DOX-induced cardiotoxicity.